A MTJ memory element is also referred to as a MTJ nanopillar or MTJ, and is a key component in magnetic recording devices, and in memory devices such as magnetoresistive random access memory (MRAM) and spin torque transfer (STT)-MRAM. An important step in fabricating an array of MTJs is etch transfer of a pattern in an overlying hard mask through a MTJ stack of layers to form an array of MTJs with a critical dimension (CD) that in state of the art devices is substantially less than 100 nm from a top-down view. The etch transfer process typically involves a plurality of etch steps involving reactive ion etch (RIE) and/or ion beam etch (IBE) and stops on a substrate which is generally a bottom electrode.
A MTJ stack of layers includes two ferromagnetic layers called the free layer (FL) and reference layer (RL), and a dielectric layer (tunnel barrier) between the FL and RL. The RL has a fixed magnetization preferably in a perpendicular-to-plane direction (perpendicular magnetic anisotropy or PMA) while the FL is free to rotate to a direction that is parallel (P) or anti-parallel (AP) to the RL magnetization direction thereby establishing a “0” or “1” memory state for the MTJ. The magnetoresistive ratio is expressed by dR/R (or DRR) where dR is the difference in resistance between the two magnetic states (RAP−RP) when a current is passed through the MTJ, and R=RP is the minimum resistance value.
The bottommost MTJ layer is usually a non-magnetic seed layer that promotes uniform growth in overlying layers, and enhances PMA in the overlying RL or FL. A capping layer such as Ta is generally formed as the uppermost MTJ layer and serves as a protective layer during subsequent physical and chemical etches. Thus, a single etch transfer process through the MTJ stack of layers is challenging since there are a variety of materials (magnetic alloys, non-magnetic metals, and dielectric films) that each have a different etch rate when subjected to IBE with Ar or to conventional CH3OH based RIE. In particular, methanol RIE causes chemical and plasma damage on MTJ sidewalls although there is minimal redeposition of etched material on the sidewalls. On the other hand, IBE produces no chemical damage and leaves minimal plasma damage, but results in a high degree of redeposited material on MTJ sidewalls. In both RIE and IBE, a so-called dead layer is formed on MTJ sidewalls and is comprised of one or both of redeposited material and damaged material including oxidized portions of MTJ layers. When the dead layer includes one or more metals from the hard mask or another MTJ layer, or from the bottom electrode, and is formed on the tunnel barrier, an electrical shunt or “short” may easily occur and render the device unusable.
An electrical shunt is often observed as a “low tail” population in a plot of DRR vs. resistance (RP) as shown in FIG. 1. The cluster 3 of data points outside the main population 2 and spreading toward zero DRR and zero RP is defined as the “low tail”. MTJs with this low tail population are undesirable for STT-MRAM applications since they have a small DRR as well as low RP. This result occurs because as the electrical short (shunt) becomes larger, more current passes through the shunt pathway and does not contribute to tunnel magnetoresistance.
Current technology does not provide a single etch solution for transferring a hard mask pattern through an entire stack of layers without either a substantial redeposition of one or more MTJ materials on the MTJ sidewalls, or significant damage to the sidewalls. In any case, removal of material from the sidewalls requires one or more extra steps that reduce throughput and add cost. Moreover, damaged sidewalls are difficult to repair and often lead to reduced yield and therefore higher cost per unit of acceptable product. Therefore, a new method for etching a MTJ stack of layers in a single etch process is needed for higher throughput and lower cost, and the method must maintain or preferably improve magnetic properties including increasing DRR and decreasing the low tail population in the resulting plurality of MTJs. Furthermore, a process flow for etching MTJ sidewalls is desired that substantially reduces sidewall residue for devices with a diameter (CD) around 60 nm or less.